Close the graph editor. You'll notice that your box will have vanished. Thats because the master channel starts at a value of 0. So your box is currently at 0 Y and Z scale. Not to worry. Click the Box Sliders tag in the viewport to bring up the sliders. Open the properties and double click the sliders plug to open the slider panel. Expand the MC set in the slider panel and choose the Box YZ scale master channel. Click add channel to add it to your slider panel. Leave the min max and label setting at the default. Hit continue to close the slider panel.

Set the MC:Box YZ Scale slider to 1 to bring your box back. Play with the sliders to see the different effects. Test out what the other controls on the slider title do.

I hope this tutorial was helpful in getting you going on using sliders. Theres a lot of other tricks you can do with sliders and hopefuly I'll have time to make up a nice tut for them too.